1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of the Related Art
For a field-effect transistor for use in a related-art large scale integrated circuit (LSI) apparatus, miniaturization of elements has been advanced in order to increase the operation speed and to reduce power consumption. As a method of miniaturizing an element, especially the effective thickness of the gate insulation film has been reduced.
However, with the reduction of the thickness of the silicon oxide film which has heretofore been used in the gate insulation film, a problem occurs that a tunnel current flows through the film. To solve the problem, a metal silicate material, which is high in relative dielectric constant compared with silicon oxide, has been used in the gate insulation film. For the gate insulation film formed of the metal silicate material, even with about 1 nm in terms of an equivalent silicon oxide film thickness, the actual physical film thickness can be sufficiently large to prevent the tunnel current.
Moreover, to increase the sophistication of the integrated circuit while keeping the power supply voltage constant, there is a method of driving transistors constituting an input/output circuit at a high voltage; and driving transistors constituting an inner circuit other than an input/output portion at a low voltage. Therefore, an LSI apparatus has been proposed in which the transistors constituting the input/output circuit each includes a gate insulation film formed of a silicon oxide based material and in which the transistors constituting the inner circuit each includes a gate insulation film containing a dielectric material higher in dielectric constant than the silicon oxide based material (e.g., see Jpn. Pat. Appln. KOKAI Publication No. 2000-307010).
As described above, in the LSI apparatus, field-effect transistors connected to various load capacitances are integrated. However, in the related-art LSI apparatus, in the internal circuit which actually performs a calculation process, for both the field-effect transistors connected to a relatively large load capacitance and a relatively small load capacitance, the gate insulation film is formed of the same material. Therefore, performance of the whole LSI apparatus cannot be enhanced.
Under these circumstances, there has been a demand for realization of a semiconductor device in which a plurality of transistors having appropriate driving forces in accordance with a size of the load capacitance can be integrated as the internal circuit on one semiconductor substrate.